1. Field of the Invention
This invention relates to solder joints that electro-mechanically connect wires and conductive pads and more specifically to a strap device and method for clamping soldered wires to improve the reliability of solder joints.
2. Description of the Related Art
Soldered wires are commonly used to provide electrical interconnections. In general, the stranded or solid wire is soldered flat to a conductive pad on a substrate. For example, in solar arrays, soldered wires are used to connect the series connected solar cell strings in series or parallel. The solar cell strings and their end termination tabs are mounted on a solar panel and the soldered wires are spot bonded to the panel to hold them down. Soldered wires are also used to connect the circuity on a printed circuit (PC) board to external circuitry. The soldered wires are terminated at a power bus on the PC board. In both of these cases, there is typically a substantial mismatch between the thermal coefficients of expansion (TCE) for the conductive pad (end termination tab or power bus) and the substrate (solar panel or PC board). Furthermore, there is also a mismatch between the solder and the copper wire/conductive pad.
When the soldered wires are subjected to extreme temperature cycling, such as found in satellites, the solder joints will become fatigued over time. A satellite in geo-synchronous orbit may, for example, cycle between -180.degree. C. and +80.degree. C. for 1600 cycles and in low earth orbit may cycle between -80.degree. C. to +100.degree. C. for 30,000 cycles. The TCE mismatch causes the substrate, conductive pad, solder and wire to expand and contract at different rates. This produces mechanical stress on the solder joint. Furthermore, at the high temperature end of the thermal cycle, the solder will soften and the flexing wire will try to pull up and off of the conductive pad. As the solder joint becomes fatigued it becomes more susceptible to cracking and eventually springing off the pad thereby breaking both the mechanical and electrical connections.
To reduce the stress on the solder joints, the wires are typically bent into an S-shape between two solder joints on the surface of the substrate. As a result, the force exerted on the solder joints as the wire expands and contracts is reduced. Although this improves the reliability of the solder joint, it may still fail.
Where reliability specifications require a redundant mechanical attachment in case the solder joint should fail, a pull-tab is cut into the conductive pad, punched to form a hole in the pull-tab, and bent upwards. The wire is inserted into the hole and soldered. If the solder joint fails, the pull-tab is supposed to maintain the mechanical and electrical connection to the wire. However, cutting the pull-tab out of the conductive pad weakens the pad itself. Furthermore, the pull-tab is prone to tearing along its bottom edge where it is connected to the conductive pad. Lastly, the solder joint around the circumference of the wire is only as thick as the pull-tab. Because the pull-tab is typically very thin, approximately 0.05 cm, the solder joint is relatively weak.